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Abstract:

A current limiter circuit in an IC having a power transistor and an output
current detection circuit connected in series with the power transistor.
A comparator, a first reference voltage generator circuit and a second
reference voltage generator circuit are also included in the current
limiter circuit. The comparator generates a control signal for stopping a
drive of the power transistor for a predetermined time. The comparator
generates the control signal according to a detection signal obtained by
the output current detection circuit and a second reference voltage
obtained by the second reference voltage generator circuit when the
output current of the power transistor reaches a predetermined value
larger than the predetermined limit value. The first reference voltage
generator circuit is provided externally of the IC and the second
reference voltage generator circuit is included within the IC.

Claims:

1. A current limiter circuit in an IC including a power transistor and an
output current detection circuit, said current limiter circuit
comprising:a comparator;a first reference voltage generator circuit; anda
second reference voltage generator circuit,wherein said output current
detection circuit connected in series with said power transistor, said
comparator generates a control signal for stopping a drive of said power
transistor for a predetermined time according to a detection signal
obtained by said output current detection circuit when an output current
of said power transistor reaches a predetermined limit value and
according to a first reference voltage obtained by said first reference
voltage generator circuit, and said comparator generates the control
signal according to a detection signal obtained by said output current
detection circuit when the output current of said power transistor
reaches a predetermined value larger than the predetermined limit value
and according to a second reference voltage obtained by said second
reference voltage generator circuit, said first reference voltage
generator circuit is proved externally of said IC and said second
reference voltage generator circuit is included within said IC.

2. The current limiter circuit as claimed in claim 1, wherein said second
reference voltage is set within a range in which said power transistor
can continuously operate as a driver.

3. The current limiter circuit as claimed in claim 2, wherein the output
current of said power transistor is outputted as a drive current of a
motor.

4. The current limiter circuit as claimed in claim 3, wherein the output
current of said power transistor is a sink drive current from an output
terminal to which the output current of said power transistor is
outputted.

5. The current limiter circuit as claimed in claim 4, wherein the
predetermined value is in a range larger than the predetermined limit
value by 3% to 10% of the predetermined limit value, said output current
detector circuit includes a resistor externally provided through said IC
and the detection signal is a terminal voltage generated by said
resistor.

6. The current limiter as claimed in claim 5, further comprising a
chopping pulse generator circuit and a timer circuit, wherein the
predetermined time period is a constant time period, said timer circuit
clocks the constant time period in response to the control signal, said
chopping pulse generator circuit generates pulse with an interval of the
constant time period set by said timer circuit and said power transistor
is ON/OFF controlled according to the pulses.

7. A motor drive circuit for driving a motor by the output current of said
power transistor of said IC including said current limiter circuit
claimed in claim 1.

8. The motor drive circuit as claimed in claim 7, wherein said motor is a
stepping motor.

Description:

TECHNICAL FIELD

[0001]The present invention relates to a current limiter circuit and a
motor drive circuit and, in particular, the present invention relates to
a current limiter circuit capable of preventing over current when an
external reference voltage generator circuit, which detects a standard
current value (limited current value), malfunctions in a stepping motor
driver IC of unipolar (half wave) drive and protecting power transistor s
so that the driver IC can be continuously used.

BACKGROUND ART

[0002]In a stepping motor driver (pulse motor driver) of unipolar drive, a
gear-shaped rotor is rotated by a predetermined angle by sequentially
exciting a stator of the stepping motor by a single phase drive, a single
phase-two phase drive or a two phase drive, etc.

[0003]The driver for supplying drive current for exciting the stator
sequentially includes coils (exciting coils), which are provided on the
stator and connected to a power source line, and power transistors
(output stage transistors), which are provided for respective phases and
connected in series with the respective coils. The stepping motor is
driven by sequentially exciting the stator by ON/OFF control of the power
transistors with a predetermined timing.

[0004]When the power transistor of a certain phase is turned ON, the drive
current is sequentially increased in the ON period due to transient
phenomenon having a predetermined time constant, which is determined by
inductance of the exciting coils in the same phase and impedance of the
power transistors in the same phase, etc. In order to limit the increase
of drive current to a predetermined value, the power transistor is turned
ON and, after a predetermined time lapses from the turning ON, turned OFF
so that an over current does not flow through the power transistor. In
order to realize the scheme, the power transistor is driven such that
each phase is chopped by logical pulses of HIGH level "H" and LOW level
"L".

[0005]As an example of such pulse drive control, a three phase motor
driver, which is chopper-controlled by setting an ON period by a timer
circuit, and a power transistor protective circuit for integrated gate
bipolar transistors (IDBTs) of the three phase motor driver are well
known (Patent Reference 1).

[0006]As described in Patent Reference 1 (JPH11-112313A), an over-current
protective circuit for such kind of driver is constructed with a current
detector circuit for detecting an output current of the power transistor
and an over-current detective circuit for stopping a drive of a power
transistor. The current detection circuit is usually provided in series
with the power transistor. The over-current detection circuit is
activated in response to a detection signal from the current detection
circuit, which is obtained when the output current of an output stage
power transistor becomes larger than the predetermined value, to limit
the output current.

[0007]Patent Reference 1: JPH11-112313A

DISCLOSURE OF THE INVENTION

Problems that the Invention is to Solve

[0008]In general, a comparator compares a voltage signal from the current
detector circuit with a reference voltage and, when the voltage signal
exceeds the reference voltage, a current limiter circuit stops the drive
of the power transistor. When a circuit, which generates the reference
voltage, malfunctions, the current limiter circuit does not work and the
power transistor may be broken. Therefore, an over-current protective
circuit is required separately.

[0009]The reference voltage generator circuit for detecting a rated
current (limit current value) by means of the current limiting circuit is
provided externally of the driver IC. This is because the voltage for
detecting the rated current value varies correspondingly to variation of
the characteristics of power transistor and, so, it is necessary to
regulate the limit current to a value inconformity with a design
specification by regulating the voltage value by means of the externally
provided reference voltage generator circuit.

[0010]Therefore, defective connection of not the circuits within the
driver IC but the circuits provided externally of the driver IC tends to
occur. When the reference voltage input terminal becomes open by such
defective connection, the current limiting circuit does not work and the
power transistor becomes ON. The over-current protective circuit provided
separately may detect an over-current of the output current, which flows
when the power transistor is ON. However, the over-current protective
circuit can not be continuously used as the driver since the operation of
the driver IC is stopped. Particularly, in the driver of such as the
motor drive circuit, it does not work as the driver due to a mere
malfunction of the circuit for generating the reference voltage and the
motor also does not work. Therefore, there is a problem that a whole
system or device may become useless.

[0011]The present invention is intended to solve the problem of the prior
art and an object of the present invention is to provide a current
limiting circuit or a motor drive circuit, which protects a power
transistor by preventing over-current from occurring and can be
continuously used as a driver IC, when an externally provided reference
voltage generator circuit for detecting a rated current malfunctions.

Means for Solving the Problems

[0012]In order to achieve the above object, a current limiting circuit or
a motor drive circuit according to the present invention includes an
output current detector circuit connected in series with each power
transistor, a comparator, a first reference voltage generator circuit and
a second reference voltage generator circuit. When an output current of
the power transistor becomes a predetermined limit value, the comparator
generates a control signal for stopping a drive of the power transistor
for a predetermined period on the basis of a detection signal obtained by
the output current detection circuit and a first reference voltage
obtained by the first reference voltage generator circuit and, when the
output current of the power transistor becomes a predetermined value
exceeding the predetermined limit value, the comparator generates a
control signal on the basis of a detection signal of the output current
detector circuit and a second reference voltage of the second reference
voltage generator circuit. The first reference voltage generator circuit
is provided externally of the driver IC and the second reference voltage
generator is provided within the driver IC.

Advantage of the Invention

[0013]In the present invention, the second reference voltage generator
circuit is provided within the driver IC. Therefore, when the first
reference voltage generator circuit for detecting the rated current value
malfunctions, the second reference voltage generator circuit limits the
current to prevent over-current from flowing and to protect the power
transistor.

[0014]There may be substantially no defective connection in the second
reference current generator circuit provided within the driver IC.
Therefore, the driver IC can be reliably protected. By setting the above
mentioned predetermined value to a level, which is slightly higher than
the first reference voltage and does not cause any problem in
continuously operating as the driver IC, the operation of the driver IC
has no adverse effect.

[0015]Therefore, it becomes possible to continuously use the IC as the
driver even when the externally provided first reference voltage
generator circuit is not exchanged.

[0016]Incidentally, since the voltage to be generated by the first
reference voltage generator circuit can be easily determined, provided
that the voltage to be generated through a connecting terminal thereof
can be checked, it is easy to recover the normal operating state by
exchanging the first reference voltage generator circuit.

[0017]In such case, the voltage to be generated by the first reference
voltage generator circuit can be easily obtained from the voltage of the
second reference voltage generator circuit.

[0018]The rated current value corresponds to a limit current (design
value) for not the over-current protection but limitation of the current
below a certain current when the motor drive circuit is chopper-driven.
The voltage of the second reference voltage generator circuit is used for
both the over-current protection and the current limitation. The
over-current protective circuit is primarily provided for preventing the
IC from being broken. However, by setting the voltage of the second
reference voltage generator circuit close to the limit current caused by
the voltage of the first reference voltage generator circuit, the
over-current protection circuit works as a current limiter circuit when
the current limiting operation by the voltage of the first reference
voltage generator circuit becomes impossible.

[0019]Incidentally, the voltage close to the limit current may be higher
than an upper limit value of a voltage variation of the externally
provided first reference voltage generator circuit and equal to or lower
than the maximum rated current of the power transistor.

[0020]That is, the limit current caused by the voltage of the second
reference voltage generator circuit exceeds the rated current and is in a
range in which there is no problem even when the power transistor
continues the motor drive operation. For example, the limit current
caused by the voltage of the second reference voltage generator circuit
is preferably higher by 3% to 10% of the current for limiting the
current.

[0021]As a result, the driver or the motor is not influenced by only
malfunction of the reference voltage generator circuit and it is possible
to prevent the mechanism or the whole device from being damaged.

BEST MODE FOR CARRYING OUT OF THE INVENTION

[0022]FIG. 1 is a block circuit diagram of a single phase drive circuit of
a unipolar drive stepping motor driver using a current limiter circuit
according to an embodiment of the present invention and FIG. 2 is a
circuit diagram of a comparator of the current limiter circuit.

[0024]The current output circuits 1a to 1d have identical circuit
constructions and, therefore, only the current output circuit 1a is shown
and described in detail. Incidentally, a reference numeral 12 depicts a
power source.

[0025]The current output circuit 1a includes an N channel MOSFET power
transistor Tr. A drain of the power transistor Tr is connected to an
output terminal 2a and an exciting current is outputted to the output
terminal 2a. A source of the power transistor Tr is connected to a
resistor Rs for detecting an output current. The resistor Rs is provided
externally of the stepping motor driver IC and grounds a terminal 2e. An
output current of the output terminal 2a is a sink current from the
exciting coil 11a. The current limiter circuit 3 includes a doubling
(×2) amplifier 4, a comparator 5, a first voltage generator circuit
6a and a second voltage generator circuit 6b.

[0026]The amplifier 4 is connected between the terminal 2e and a (-) input
terminal of the comparator 5. The reference voltage generator circuit 6a
is provided externally of the stepping motor driver IC and connected to a
(+) input of the comparator 5 through a terminal 2f. Thus, the reference
voltage generator circuit 6a functions to apply a reference voltage VREF
to the (+) input of the comparator 5. On the other hand, the reference
voltage generator circuit 6b is provided within the stepping motor driver
IC and connected to other (+) input of the comparator 5 to apply a
reference voltage VR (VR>VREF) to the (+) input of the comparator 5.

[0027]Incidentally, the reference voltage VR is slightly higher than the
reference voltage VREF to avoid problem when the stepping motor driver IC
is operated with this reference voltage VR.

[0028]The reference voltage VR is close to a voltage corresponding to the
limit current value caused by the reference voltage VREF. The voltage VR
is determined such that the current is limited to a value larger by 3% to
10% of the designed limiting current. It is enough that the voltage close
to the limit current may be higher than an upper limit value of a
variation of the reference voltage VREF of the externally provided first
reference voltage generator circuit 6a and equal to or lower than the
maximum rated current of the power transistor.

[0029]When the output current of the power transistor Tr increases and a
drive current (output current), with which a terminal voltage Vs of the
resistor Rs for detecting the output current exceeds the reference
voltage VREF, is generated in the power transistor Tr, that is, when the
output current becomes the predetermined limit value (limit current
value), the output of the comparator 5 is changed from "H" to "L",
resulting in a detection pulse S ("L" is significant). The detection
pulse S is supplied to an internal delay circuit 7 and a delayed
detection pulse S is inputted to a clock terminal CLK of an RS-flip-flop
(data latch circuit) 8 as a fall-trigger signal. At this time, 1-bit data
of the detection pulse S ("L"), which is not delayed, is supplied to a D
terminal of the RS-flip-flop. Therefore, the 1-bit data is latched by the
delayed trigger signal.

[0030]As a result, the output of the RS-flip-flop 8 becomes "L", which is
supplied to an AND gate 9.

[0031]A phase exciting signal G ("H") from a phase exciting signal
generator circuit (not shown) is supplied to the AND gate 9. Thus, the
AND gate 9 is closed by the output ("L") of the RS-flip-flop 8. As a
result, the phase exciting signal G ("H") supplied to a gate of the power
transistor Tr is blocked and the power transistor Tr becomes OFF. When
the power transistor Tr becomes OFF, the voltage Vs becomes ground
potential and the output (detection pulse S) of the comparator 5 is
changed from "L" to "H", so that the detection pulse S is ended.

[0032]Therefore, the detection pulse S operates as a control signal for
turning the power transistor Tr OFF.

[0033]On the other hand, the detection pulse S ("L") is also supplied to a
timer circuit 7a, which generates a chopping pulse to the RS-flip-flop 8
after a constant time from the input of the detection signal. That is,
after the constant time from a time when the power transistor Tr is
turned OFF, the chopping pulse P ("H") is supplied to the internal delay
circuit 17 through the timer circuit 7a and an inverter 7b. Further, the
chopping pulse P ("H") is supplied to the D terminal of the RS-flip-flop
8 without delay.

[0034]The internal delay circuit 7 generates a trigger pulse, which falls
when the chopping pulse P rises. Therefore, "H", that is, "1" is latched
by the RS-flip-flop 8 during the chopping pulse P is "H", so that the
phase exciting pulse G having a quiescent time corresponding to a time
count of the timer circuit 7a is generated at a Q output of the RS
flip-flop 8. As a result, the AND gate 9 is opened. Thus, the AND
condition is established when the phase exciting signal G is "H" and the
power transistor Tr supplies an increasing drive current to the exciting
coil 11a. When the amount of the drive current reaches the predetermined
limit value (limit current value), the output of the comparator 5 is
changed from "H" to "L" and the detection pulse S is generated. Thus, the
power transistor Tr is turned OFF again.

[0035]By repeating this operation, the output current of the power
transistor Tr is chopped during the drive period in which the phase
exciting signal G ("H") is supplied to the gate of the transistor Tr and
the drive current flows to the exciting coil 11a correspondingly to the
timing of the generation of the phase exciting signal G.

[0036]Incidentally, the timer circuit 7a functions to change the chopping
pulse P, which is in H level, to L level for a constant time. When there
is no detection pulse S supplied, the timer circuit 7a generates the
chopping pulse P in "H" level to thereby set "1" in the RS-flip-flop 8
and hold the AND gate 9 opened. The AND condition is established when the
phase exciting signal G ("H") is generated and the power transistor Tr
supplies the drive current to the exciting coil 11a. The above mentioned
operation is started correspondingly to the generation of the phase
exciting signal G.

[0037]Thus, the current limiter circuit 3 limits the output current of the
power transistor Tr by blocking the drive current when the voltage Vs of
the resistor Rs at a terminal 2e exceeds the reference voltage VREF, that
is, when the output current of the power transistor Tr becomes the rated
current value. In this point, the current limiter circuit 3 serves as
both the current limiter and the over-current protective circuit.

[0038]It is assumed that the reference voltage VREF does not appear at the
terminal 2f by malfunction of the reference voltage generator circuit 6a
or defective connection of the terminal 2f.

[0039]In such case, the output current of the power transistor Tr
increases and the voltage Vs exceeds the reference voltage VREF. When the
output current, with which the voltage Vs exceeds the reference voltage
VR, is generated in the power transistor Tr, that is, when the output
current becomes a predetermined value equal to or larger than the
predetermined limit value, the comparator 5 outputs the detection pulse S
("L" is significant) which is changed from "H" to "L".

[0040]That is, a comparative reference voltage of the comparator 5 is
changed from the reference voltage VREF of the reference voltage
generator circuit 6a to the reference voltage VR of the reference voltage
generator circuit 6b and the above mentioned operation is continuously
performed. Therefore, the operation of the stepping motor driver IC 10 as
the driver can be continued.

[0041]FIG. 2 is a circuit diagram showing the comparator 5. The comparator
5 includes a differential amplifier 50 composed of PNP transistors Q1 and
Q2. Emitters of PNP transistors Q3 and Q4 are connected in parallel to a
base of the transistor Q1. Collectors of the transistors Q3 and Q4 are
grounded.

[0042]An emitter of a PNP transistor Q5 is connected to a base of the
transistor Q2 and a collector of the PNP transistor Q5 is grounded. A
current detection signal from the doubling (×2) amplifier 4 is
supplied to the base of the PNP transistor Q5.

[0043]The reference voltage generator circuit 6a is provided between the
base of the transistor Q3 and the terminal 2f and the reference voltage
generator circuit 6b is provided between the base of the transistor Q4
and ground (GND).

[0044]Reference numerals 51 to 53 are current sources provided between the
emitters of the respective transistors Q1 to Q5 and a power source line
VDD. NPN transistors Q6 and Q7 constituting a current mirror circuit are
provided downstream side of the transistors Q1 and Q2 as an active load
circuit of the differential amplifier 50. Emitters of the transistors Q6
and Q7 are grounded.

[0045]NPN transistors Q8 and Q9 are output stage transistors having
emitters grounded. A collector of the transistor Q8 is connected to the
power source line +VDD through a current source 54 and an output of the
collector of the transistor Q6 is supplied to a base of the transistor
Q8. A collector of the transistor Q9 is connected to the power source
line +VDD through a load resistor R. A base of the transistor Q9 receives
the collector of the transistor QB to generate the detection pulse P.

[0046]The voltage to be generated by the first reference voltage generator
circuit 6a can be easily known by checking it through the terminal 2f.
Therefore, when the first reference voltage generator circuit 6a
malfunctions and is replaced by another first reference voltage generator
circuit, it is easily possible to recover the normal operation.

[0047]The voltage to be generated by the first reference voltage generator
circuit 6a may be equal to the voltage of the second reference voltage
generator circuit 6b or lower than the voltage of the second reference
voltage generator circuit 6b by a predetermined value. Therefore, it is
better to employ a circuit construction in which the voltage of the
second reference voltage generator circuit 6b is outputted to the
connecting terminal (terminal 2f) of the first reference voltage
generator circuit 6a. The terminal 2f in FIG. 2 is used in such circuit
construction. When the voltage of the second reference voltage generator
circuit 6b is higher than the reference voltage VREF of the first
reference voltage generator circuit 6a by 1 Vf (forward dropdown voltage
between the base and the emitter) or more, the transistor Q4 is kept OFF
so long as the first reference voltage generator circuit 6a is connected
to the terminal 2f.

[0048]Assuming that the predetermined limit value of the output current of
the power transistor Tr, which is limited by the reference voltage VREF
of the reference voltage generator circuit 6a, is 2.6 A, the output
current of the power transistor, which is limited by the voltage VR of
the reference voltage generator circuit 6b, is set to about 2.7 A
(=2.6×1.038), which is not detrimental for the circuit operation.
There is no need of changing the circuit relation as the current limiter
circuit. Incidentally, the maximum rated current of the power transistor
Tr is 3.0 A (>2.6 A).

[0049]As a result, when the reference voltage generator circuit 6a
malfunctions and the reference voltage VREF can not be applied to the
comparator 5, the voltage VR slightly higher than the reference voltage
VREF is set so that the operation of the driver IC is kept and can be
continuously operated as the driver.

[0050]In the described embodiment, the comparator 5 is provided in each of
the current output circuits 1a to 1d. However, it may be possible that a
plurality of power output circuits commonly use a current output circuit.
In such case, it is possible to use 2 comparators by making the output
current detection resistors Rs of the comparators 5 of the current output
circuits 1a and 1b common and the output current detection resistors Rs
of the comparators 5 of the current output circuits 1c and 1d common.

[0051]Although the power transistor Tr is the MOSFET in this embodiment, a
bipolar transistor may be used.

[0052]Further, although, in the embodiment, the motor drive circuit of the
unipolar drive stepping motor driver IC is described, it is possible to
use a push-pull drive circuit as the output circuit of the power
transistor and to apply this invention to a bipolar drive (positive and
negative phase drive) stepping motor driver IC.

INDUSTRIAL APPLICABILITY

[0053]Although, in the described embodiment, the power transistor is
ON/OFF controlled by the internal delay circuit 7, the RS-flip-flop (data
latch circuit) 8, the AND gate 9 and the OFF timer circuit 7a, these
circuits are not always necessary so long as the power transistor Tr is
OFFed.

[0054]Further, although, in this embodiment, the comparator 5 has two (+)
input terminals, it is possible to constitute the comparator 5 with two
parallel comparators. Alternatively, it is possible to provide two
parallel comparators each having a (+) input and a (-) input.

[0055]Although the present invention has been described with reference to
the stepping motor driver circuit, the present invention can be applied
to any drive circuit including a current limiter circuit or an
over-current protective circuit for limiting a drive current by turning a
power transistor OFF with a rated current value.

BRIEF DESCRIPTION OF THE DRAWINGS

[0056]FIG. 1 is a block diagram showing a unipolar drive circuit of a
stepping motor driver having a current limiter circuit according to an
embodiment of the present invention.

[0057]FIG. 2 is a circuit diagram of a comparator in the current limiter
circuit.